Constant on-time DC/DC converters are widely used in power conversion due to their excellent load transient response, high efficiency, simple configuration and small size. One example is the application in consumer electronics such as notebook, netbook, computer, and personal digital assistant (“PDA”).
FIG. 1 schematically illustrates a traditional DC/DC converter. For constant on-time control, when output voltage of the converter is lower than a reference voltage, the converter is configured to provide power from input terminal to output terminal. As one example shown in FIG. 1, when output voltage is less than a reference voltage, a high-side switch S1 is turned on and a low-side switch S2 is turned off, then a current is supplied from input terminal VIN to output terminal VO. Output voltage is obtained at output terminal VO through an output filter comprising an inductor L and an output capacitor C. Output capacitor C comprises an ideal capacitor CO and an equivalent series resistance (“ESR”).
FIG. 2 and FIG. 3 show waveforms illustrating signals of the converter shown in FIG. 1. The first waveform shows a driving signal 201/301 of high-side switch S1. The second waveform shows a voltage ripple 202/302 across the ESR of output capacitor C. The third waveform shows a voltage ripple 203/303 across the ideal capacitor CO of output capacitor C. And the last waveform shows an output voltage ripple 204/304. FIG. 2 shows waveforms when the resistance value of the ESR is relatively small. Voltage ripple 202 across the ESR is in phase with driving signal 201. The ideal capacitor CO has an integral effect and voltage ripple 203 across the ideal capacitor CO is 90 degree delayed to driving signal 201. Since the resistance value of the ESR is relatively small, voltage ripple 203 across the ideal capacitor CO plays a dominant role, and output voltage ripple 204 is determined by the ideal capacitor CO. Thereby output voltage ripple 204 is delayed to driving signal 201, and the system may be unstable as shown in FIG. 2. When the resistance value of the ESR is relatively large, voltage ripple 302 across the ESR plays a dominant role. As shown in FIG. 3, output voltage ripple 304 is about in phase with driving signal 301, and the system is stable. In summary, an equivalent series resistance with large resistance value is needed in the traditional constant on-time converter to stabilize the system.
Accordingly, a constant on-time converter with good stability is needed, especially when output capacitor with low ESR is used, e.g., ceramic capacitor.